invention relates to a process for the preparation of 4,4'-dinitrostilbene-2,2'-disulphonic acid salts by the oxidation of 4-nitrotoluene-2-sulphonic acid or salts thereof in an aqueous alkaline medium.
The preparation of 4,4'-dinitrostilbene-2,2'-disulphonic acid or salts thereof (described as dinitrostilbene acid below) has been known for a long time and consists in subjecting 2 moles of 4-nitrotoluene-2-sulphonic acid (described as p-nitrotoluenesulphonic acid below) to an oxidative condensation reaction under aqueous alkaline conditions. Oxygen (air), in most cases in the presence of a catalyst, or sodium hypochloride or chlorine have been employed as oxidizing agents (see, for example, F. Bender and G. Schultz, .Ber., 19, 3234; 0. Fischer and E. Hepp, Ber., 26, 2233; and DE-OS (German Published Specification) No. 2,258,530). Because of the poor yields of only approximately 75% in the known industrial processes, attempts have been made to increase the yields of dinitrostilbene acid by varying the reaction conditions.
Thus, in accordance with USSR Patent Specification No. 230,136, the sodium hydroxide solution is added in two stages at different concentrations (approximately 2% strength sodium hydroxide solution in the first stage and approximately 4% strength sodium hydroxide solution in the second). The yield of dinitrostilbene acid is, however, only 65% of theory at most.
East German Patent Specification No. 204,272 describes a process for the preparation of dinitrostilbene acid by the oxidation of p-nitrotoluenesulphonic acid in an aqueous alkaline medium by means of atmospheric oxygen, which is characterized in that, the higher the alkali concentration and/or the reaction temperature, the lower the concentration at which the p-nitrotoluenesulphonic acid is kept in the oxidation solution. The yields of dinitrostilbene acid are quoted as 70 to 80% of theory.
Polish Patent Specification No. 64,173 discloses a process in which the oxidation is carried out in a 4-plate column. The oxygen-air mixture is fed, at an oxygen content of 20 to 50% by volume, counter-current in a closed circuit. A concentration range from 3.1 to 10% by weight is quoted for the sodium hydroxide solution and a range from 4 to 8% by weight is quoted for the sodium salt of p-nitrotoluenesulphonic acid. The yields are 60 to 85% of theory, depending on the concentrations of the reactants.
An increase in the yield of dinitrostilbene acid to a maximum of 82% of theory is described in Japanese Patent Specification No. 8,238,764. Described as well as the catalyst are manganese sulphate, compounds of molybdenum barium. iron, nickel and cobalt or mixtures thereof.
A process for the preparation of dinitrosilbene acid and salts thereof by the oxidation of p-nitrotoluenesulphonic acid in organic solvents is described in European Offenlegungsschrift (European Published Specification) No. 26,154. Yields of up to 95% of theory are achieved in this process, depending on the mode of operation. Disadvantageous factors in this process are carrying out the reaction in organic solvents and the low reaction temperatures.
British Patent Specification No. 2,136,430 discloses a process for the preparation of dinitrosilbene acid, which is characterized in that the oxidation of p-nitrotoluenesulphonic acid is carried out in the presence of lithium and hydroxyl ions, if appropriate with the addition of a catalyst. The yields in this process are quoted as about 80 to 90% of theory. The removal of lithium carbonate before the isolation of the dinitrostilbene acid is necessary in this process as an additional stage. The recovery of lithium carbonate is only 75%, however. Additionally, the lithium carbonate removed must first be converted into lithium hydroxide before it can be re-used in the process.
As a result of the removal, which is, above all, expensive, of the lithium carbonate before the isolation of the dinitrosilbene acid, the process described in the British Patent Specification does not prove very economical.
Disadvantageous factors in the processes known hitherto for the preparation of dinitrosilbene acid are not only the yields, which, all things considered, are unsatisfactory, but also, in addition, the formation of undesirable by-products. These pollute the effluent and thereby cause considerable expense.